This invention relates to a process for dissolving plutonium dioxide and/or neptunium dioxide in acid, particularly nitric acid.
One of the problems encountered in manufacturing of nuclear fuel and the reprocessing of irradiated fuel involving plutonium dioxide is the extreme difficulty of dissolving refractory forms of this oxide. This problem is usually encountered when trying to dissolve spent nuclear fuel which was originally fabricated from mixed oxides, i.e., UO.sub.2, the recovery of scrap during fabrication recovery from waste products such as incinerator ash, or when dissolving PuO.sub.2 in order to fabricate mixed oxide fuel by the co-precipitation method. Although PuO.sub.2 is difficult to dissolve, UO.sub.2 dissolves readily in nitric acid.
Mixed oxide fuels may be fabricated by various methods. For example, the oxides may be combined by mechanically blending of the oxides or co-precipitation of the constituent oxides. In order to fabricate mixed oxide fuel by co-precipitation, PuO.sub.2 may first be dissolved and then precipitated. The mixed oxide fuel is used in the core of nuclear reactors just as is UO.sub.2 nuclear fuel containing U(235). During operation of the nuclear reactor, the Pu(239) and U(235) fission forming numerous fission products, which include a number of strong absorbents for neutrons which interfere with the nuclear reactions (neutron poisons). If, as is usually the case, U(238) is present in the fuel and/or in blankets, Pu(239) is formed. On continued exposure, the Pu(239) is converted to higher isotopes of plutonium, including Pu(241) some of which decays by beta decay, forming americium, Am(241). The fuel must be reprocessed to remove the fission products and recover uranium and plutonium. In order to reprocess the fuel by, say, the Purex process, the PuO.sub.2 must again be dissolved, together with the UO.sub.2.
The dissolution of the mixed oxide material, PuO.sub.2 --UO.sub.2, is affected by several factors such as (1) the percentage of PuO.sub.2, (2) the method of fabrication, and (3) the irradiation level. It has been found that although UO.sub.2 dissolves in HNO.sub.3, PuO.sub.2 does not dissolve readily in HNO.sub.3 depending on the above factors. Usually the presence of fluoride ion is required for complete dissolution, but this, in turn, causes corrosion problems.
As discussed above, the difficulty in dissolving PuO.sub.2 in irradiated fuel is usually encountered in the dissolution of irradiated fuel that was originally fabrication from mixed oxides. However, some PuO.sub.3 that is produced by neutron absorption in U(238) during irradiation of UO.sub.2 nuclear fuel may also be difficult to dissolve.
Neptunium (237) is used to prepare plutonium (238). The latter is used as a power source, particularly in space applications. NpO.sub.2 is subjected to neutron irradiation forming PuO.sub.2. The mixture is then dissolved and the plutonium and neptunium separated chemically. Like PuO.sub.2, NpO.sub.2 is difficult to dissolve in nitric acid.
D. E. Horner et al., ORNL/TM-4716 (August 1977), summarized a large amount of work done at Oak Ridge on "Cerium-Promoted Dissolution of PuO.sub.2 and PuO.sub.2 --UO.sub.2 in Nitric Acid". They show the importance of keeping the ratio of Ce.sup.4+ /Ce.sup.3+ high, and suggest (Page 15) that this be done by "continuous reoxidation" by ozone or in an electrolytic oxidation cell in a circuit with the dissolver. They also disclosed the adverse effects of ruthenium on the PuO.sub.2 dissolution and stated (Page 29): "These results lead to the conclusion that the use of Ce.sup.4+ as a dissolution promoter for difficulty soluble irradiated fuel residues would not be feasible unless some way could be found to remove all the ruthenium prior to or during the dissolution."